If the garden has already been planted and is growing up to the edges of the garden walls, painting a continuation of the garden will create the illusion that the garden extends beyond its own boundaries. If you can paint the mural realistically enough, some people may even question where the garden ends and the mural begins. Use the same flowers that appear in the garden itself, and paint a continuation of any paths that may run through the garden.
If you have children or if your children tend to the garden, allow them to paint the mural. The mural may be made up of painted "signs" indicating what flowers are growing in the garden. Other possibilities include children's hand prints, signs indicating who tends the garden, or you may wish to let the children pick the mural subjects themselves.
Some gardens are meant for growing vegetables and fruits for human consumption. Other gardens are merely for decoration. And some gardens -- often the type of gardens that come with a wall and a gate around them -- are meant to create a fantasy world where beautiful plants grow. To emphasize the "fantasy" aspect of your garden, paint a mural that emphasizes fantasies. Include fairies, unicorns, dragons, knights and princesses, castles, dwarfs and gnomes. Support the fantasy by placing statues of garden gnomes and other creatures around the garden.
Paint a landscape that mimics the landscape that the plants in your garden would be found in. For example, if you have a desert plants garden, paint a desert landscape on the gate and the garden walls. If your garden has a great deal of tropical plants, paint a rain forest landscape. Include animals native to these areas.